Oil exploration systems use a variety of geographical location techniques in order to associate collected seismic data with known geographical positional fixes. Marine exploration systems, of course can have more difficult problems than land-based systems, since the former carry on activities away from usual formalized land markers, say hundreds of miles at sea. A promising geographical-fixing tool used by modern exploration systems involves the use of signals from over-passing TRANSIT navigation satellites. The signals are indicative of location of the navigator and are recorded as seismic data are collected.
In the usual satellite navigation system, the satellite generates and transmits message data giving its orbital position in inertial coordinates as a function of time. E.g., as the latter orbits the earth the satellite is programmed to continuously read out a message from which its position can be computed together with precision time. The satellite navigation receiver system uses the satellite message data and its measurements of the Doppler shift of the satellite signals to compute its position fix.
It is necessary, however, that all parts of the satellite navigation receiver system (including the receiver antennas, the receiver, the Doppler counters and decoding logic, the receiver-to-computer interface hardware and the computer hardware and software) be operating correctly to produce an accurate fix. Moreover, there is a need for a portable satellite signal simulator and method to check the satellite navigation receiver system in the field. The test units of which we are aware test only individual parts of the satellite navigation receiver system, namely the Doppler counters and decoding logic, and do not test the entire satellite navigation receiver system from antenna input to resulting positional fix. There are additional problems in designing such a simulator: there is a variety of satellite navigation receivers to be used in association with even more varied computer systems. Since any one receiver can be used with a plurality of computer hardward-software, design of the simulator system can be even more difficult, especially when the tests are to be performed at sea by non-programming technologists.